The objective of this study is to determine whether calcium supplementation can prevent bone loss during lactation and increase bone gain after weaning. Two cohorts of women are studied, one during the first 6 months of lactation and the other during 6 months post-weaning. Half of the women in each group are lactating and the other half are non-lactating controls. Within each lactation group, half of the women are randomized to receive 1000 mg/d calcium.